The field of the disclosure relates generally to systems and methods for transmitting direct current (DC) power to a load, and more specifically to a DC transmission system having a converter with an active alternating current (AC) stage that does not require heavy DC insulation with respect to ground and a passive AC-to-DC conversion stage.
Most land based transmission systems transmit AC power from an electrical power source, such as a generator, to an electrical load, such as a motor. Any conversion to DC power is typically performed as close to the electrical load as possible. AC power transmission systems are more prevalent than DC transmission systems due to the reduced cost and improved efficiency of AC transformers and switch gear. However, AC transmission systems have reactive losses associated with the transmission cables that do not exist in DC transmission systems. Accordingly, DC transmission systems may be more efficient and cost effective when transmitting power over long distances where the reactive losses outweigh the more expensive DC transformer equipment. This is especially true in undersea cables, where the reactive losses are compounded.
High-voltage direct current (HVDC) transmission systems and medium-voltage direct current (MVDC) transmission systems may be utilized to efficiently transmit power over long distances. However, known DC transmission systems have some inefficiencies that cause increased costs for operators. Specifically, known DC transmission systems have an AC/DC rectifier, such as a diode rectifier, that converts a received AC power, such as from the electrical grid, to DC power. The DC power is then modified by active components, e.g., DC/DC converters with active semiconductor-based devices such as insulated gate bipolar transistors (IGBTs) and thyristors. The DC power is also modified with passive components, e.g., capacitors, resistors, diodes, and transformers, to control the DC voltage level of the DC power. However, in known systems, the active components and the passive components must be heavily DC insulated to ground to prevent damage to the components in the event of a DC fault. For example, in typical systems, the active and passive components are insulated to withstand over-voltage conditions equivalent to approximately two times the transmission voltage level that may be in the ranges of tens of kiloVolts (kV) DC to hundreds of kV DC. DC insulation of passive components is relatively inexpensive, but insulation of the active DC/DC converter requires all of the auxiliary power components to be insulated with respect to ground, including gate drives and all sensors. Further, cooling systems and any other auxiliary support equipment for such active DC/DC converters also requires the same level of DC insulation with respect to ground. Accordingly, the size, amount of materials, and cost of the active components with DC insulation to ground can be significant. As some applications of DC power transmission systems have limited free space, for example, oil and gas platforms, current DC power transmission systems may be too large for a single platform, significantly increasing capital investment and operational costs.